6.9.4 Ramps and Cycles

For each of the four IRS slits (there are six ``slits'' if we count the two spectral orders available in each of the two low resolution modules), in addition to selecting which slit(s) to be used, the proposer must specify in the IRS Staring Mode AOT window two aspects of the actual exposure: the duration of ramps and the number of cycles for each ramp. The ramp duration is the time between the first and last non-destructive reads of the array. It corresponds to an individual exposure time. Ramp durations range from 6 to 480 seconds, with a limited set of discrete choices available for each IRS module. A cycle is a repetition of the exposure with all other parameters (ramp duration, slit selection, observing mode) unchanged and without moving the telescope. Since the IRS observes each ramp at two slit positions in Staring Mode, choosing a single cycle will result in two independent spectral ramps. Choosing two cycles will obtain four independent ramps, with two at the first slit position, then two at the second slit position, and so on. For our target, we determined that using exposures of $1\times120$ seconds and $1\times240$ seconds for the SH and LH slits, respectively, will result in ${\rm S/N} \approx 12$ in both sets of spectra for the target line flux of $5 \times 10^{-18} \; {\rm W \; m}^{-2}$ (see §6.7). Additional ramp cycles (e.g., $2 \times 120$ seconds) could be used to acquire spectra that could be combined to yield higher S/N. To continue the set-up for the 3C 273 observations, refer to the ``Instrument Settings'' portion of the IRS Staring Mode AOT window. Make sure that only the ``Hi Res'' slits have been checked ``On.'' Use the ``Ramp Duration'' pull-down menus to set the ramps for the SH slit (``Hi Res $9.9-19.6 \; \mu {\rm m}$'') to 120 seconds and the ramps for the LH slit (``Hi Res $18.7-37.2 \; \mu {\rm m}$'') to 240 seconds. Finally, enter the ``# of Cycles'' as ``1'' for the SH slit and ``1'' for the LH slit. The AOT window up to this point (including peak-up settings for a high accuracy, Blue filter IRS peak-up) is shown in Figure 6.12. Now, we will add the LH sky background observation. We can either add the background as a separate AOR or as a fixed cluster offset.

Figure 6.12: IRS Staring Mode AOT window for 3C 273 showing target, peak-up, slit, ramp, and cycle settings.